1. Field of the Invention
The present invention relates to a method of removing low concentration level fluorine from a gas.
2. Description of the Background
Alumina, caustic soda and soda lime are commonly used as agents for the removal of low concentration levels of fluorine from gas streams by a gas-solid reaction. For many years, removal of toxic fluorine from gas streams has been important in many industrial applications such as aluminum electrolysis furnaces, plastic fluorination processes and production of metal fluoride compounds. More recently, the increasing demand of excimer lasers in industry has led to an increased demand for the proper disposal of spent excimer gases. In some cases, spent excimer gas mixtures contain fluorine which must be removed from exhaust gas below its threshold limiting value. Typically, the venting of spent excimer gases occurs at sufficiently fast flow rates so that the space-time value in conventionally sized scrubber units is of the order of a few seconds or less. Analyses of the gaseous product resulting from this treatment, however, have shown that the removal of the low levels of fluorine can be accompanied by the formation of oxygen difluoride (OF.sub.2) in not insignificant amounts in the gas stream, in particular, when an alumina or soda-lime scrubber is used for this purpose at ambient temperature and at space-time values ranging from 0.5-3 sec, OF.sub.2 is generated as an intermediate product prior to the saturation end point where scrubber material loses its reactivity to fluorine. OF.sub.2 formation can be as high as one-third of the initial amount of fluorine under these conditions. This is a significant problem of fluorine containing exhaust streams in light of the realization that oxygen difluoride is even more toxic than fluorine. The threshold limit value (TLV) of OF.sub.2 is 50 ppb/v, whereas the (TLV) of F.sub.2 is 1 ppm/V depending on the reference. Further, the generation of OF.sub.2 in a scrubber reduces the operating lifetime of the alumina scrubber. A need therefore continues to exist for an effective means of eliminating low level concentration of fluorine from gas streams under short space-time gas flow and ambient temperatures while effectively preventing the formation of toxic OF.sub.2 by-product.